The present invention generally relates to fabrication of semiconductor devices and more particularly to a contact structure for connecting an electrode to a semiconductor device and a method of forming the same.
Connection of a wiring electrode to a semiconductor device is usually achieved through a contact hole provided on an insulator layer such as a silica glass or phosphosilicate glass (PSG) layer covering a part of the device which may be a substrate. In a typical example shown in FIG. 1, an aluminium or aluminium alloy conductor 1 is deposited on a surface of an insulator layer 2 including area of a contact hole 3 by sputtering and the like so that a part 4a of a substrate 4 exposed by the contact hole 3 at its bottom is covered by the conductor 1. When a layer of the conductor having a substantial thickness is formed continuously from the bottom of the contact hole 3 to the surface of the insulator layer 2 along a side wall 3a of the contact hole 3, there is achieved an excellent electrical contact between the substrate and the wiring electrode.
As will be easily understood, the key factor for achieving a successful electrical contact is to cover the side wall 3a of the contact hole 3 uniformly by the conductor having a substantial thickness. However, such a uniform coverage of the side wall is not achieved easily, as the side wall 3a of the contact hole generally extends vertically relative to the substrate 4 so as to reduce the diameter of the contact hole and hence the size of the device. When the conductor 1 such as aluminium or aluminium-silicon alloy is deposited by sputtering as is commonly practiced, the conductor covering the surface of the insulator layer grows laterally into the contact hole 3 and forms an overhang 1a. Under such circumstances, there is no or little deposition of the conductor at the bottom part of the contact hole 3, particularly on the side wall 3a even after a continued effort of deposition. When such a situation occurs, the layer of conductor 1 covering the side wall 3a becomes very thin as illustrated, and in the most extreme case, there is no conductor layer covering such portions. The connecting structure having such a profile is of course unstable and tends to cause disconnection of the electrical contact.
In order to eliminate the aforementioned problem, it is proposed to deposit a metal 5 such as titanium which forms a silicide when reacted with silicon, on the insulator layer 2 including the area of the contact hole 3 prior to the deposition of the conductor layer as shown by broken line in FIG. 2. The device deposited with the metal 5 is then subjected to heat treatment, whereby a layer of silicide 6 is formed so as to cover the substrate 4 in correspondence to the bottom of the contact hole 3 as a result of reaction between the metal 5 and silicon contained in the substrate 4. Further, silicon is transported upwards along the side wall 3a and there is formed a layer of silicide 6a which extends along the side wall 3a of the contact hole. After the heat treatment, the unreacted part of the metal 5 is removed. As the silicide layer 6 is formed by the reaction between the metal and silicon which is supplied from the substrate 4 through the bottom of the contact hole 3, the thickness of the silicide layer 6 is generally largest at the bottom of the contact hole 3 and the silicide layer 6a covering the side wall 3a of the contact hole 3 gradually decreases its thickness from the bottom to the top of the contact hole. In other words, the silicide layer 6 has a concaved inner surface 6b opened upwards. Further, this concaved profile of the inner surface of the silicide layer is formed with reliability even if there is formed an overhang 5a of the metal layer 5 when the metal is initially deposited on the contact hole. This is because there occurs a flow of component element constituting the metal in a direction opposite to the direction of the flow of silicon moving upwards from the substrate to the metal layer, irrespective of the initial profile of the deposited layer 5 of the metal. There is no difficulty in depositing the conductor layer on such a concaved surface 6b of the silicide layer 6 by the conventional procedure such as sputtering. By selecting the metal such that the silicide thus formed has a low resistivity, an excellent contact is achieved between the semiconductor device and the aluminium-based wiring electrode deposited on the silicide layer.
In such a prior art contact structure, there is formed a thick layer of silicide 6c at the bottom of the contact hole as already described. Such a silicide layer has a bottom surface which is generally not flat but has many projections or spikes 6d projecting into the substrate 4 particularly at a region adjacent to a bottom edge 3b of the side wall of the contact hole. Such a projection 6d is formed in a region of the substrate which acts as a source of silicon from which silicon is removed in exchange with incoming flow of the element of the metal such as titanium when the silicide layer 6a is formed along the side wall 3a of the contact hole. It should be noted that it is such a region along the bottom edge 3b of the side wall 3a that supplies most of silicon when the silicide layer 6a is formed along the side wall 3a of the contact hole 3. In conventional semiconductor devices having a relatively deep p-n junction in the substrate, the existence of such a projection of the silicide layer does not cause a serious problem. However, in a semiconductor device having a shallow junction represented schematically by a one-dotted line in FIG.2 as in the case of very large scale integrated circuits (VLSI) in which a very large number of devices are assembled in a unit area, there is a substantial risk that the spike 6d of silicide 6 thus extending into the substrate 4 causes short-circuit conduction in the shallow junction. In order to avoid such a problem and at the same time to achieve a reliable electrical contact, one has to prevent the excessive projection of the silicide into the substrate.
In a conventional contact structure where the aluminium-based wiring electrode is directly contacted with silicon substrate through the contact hole as in FIG. 1, there arises another problem of reaction between the silicon substrate and the electrode as a result of diffusion of aluminium and silicon as schematically illustrated in FIG. 1. When such a reaction occurs, a spike of aluminium silicide 4b shown by a dotted line in FIG. 1 is formed in the substrate 4 and the p-n junction in the substrate is shorted. In order to prevent such a reaction, a diffusion barrier layer (not shown) which may be a layer of titanium nitride (TiN) or titanium tungstenite (TiW) is provided between the substrate and the wiring electrode so as to block the transportation of aluminium or silicon passing therethrough. In such a conventional contact structure, there is a problem in that the coverage of the substrate at the bottom of the contact hole by the diffusion barrier layer tends to become insufficient because of the reason similar to the case of depositing the conductor layer on the substrate through the contact hole, particularly when the device has a very fine pattern and the contact hole has a correspondingly large aspect ratio which is a ratio of a depth relative to a diameter of the contact hole. This problem is further deteriorated by the limited thickness of the diffusion barrier layer as the thickness of such a diffusion barrier layer is generally limited below about 3000 .ANG. in order to secure a sufficiently low resistance of the contact structure.